1. Field of the Invention
The present invention relates to a device manufacturing method including a process of transferring a pattern to a resist film by liquid immersion lithography.
2. Description of the Related Art
With a finer pattern on a semiconductor device, improvement of an exposure tool has been advanced. There is currently used a 193 nm lithography in which an ArF laser having a wavelength of 193 nm is used as a light source. Up to now, a 157 nm lithography has been expected as a next generation lithography. However, because of a variety of factors such as a delay of the development of equipments and materials for 157 nm lithography, and a short service life, at present, 193 nm immersion lithography becomes the most promising next generation lithography (WO 99/49504 or Bruce W. Smith et al., Optical Microlithography XVII, Proceedings of SPIE vol. 5377 (2004), pp. 273-284). In the liquid immersion lithography, a refractive index of a medium fluid filled in an optical path between the final element of projection optical system and substrate stacked resist films as an imaging plane (hereinafter, referred to as a “liquid immersion medium fluid”) is large. By using a liquid immersion medium fluid with a large refractive index, a critical angle of an optical axis of the final element of projection optical system side is able to made large, so that projection of a diffractive light by a fine pattern which is smaller than a resolution limit of a ordinary exposure tool in which the medium fluid is air, becomes possible. In the liquid immersion exposure tool which is currently a mainstream of development, only an optical path between the final element of projection optical system and a substrate and a periphery of the optical path are filled by the liquid immersion medium fluid, instead of immersing a whole substrate surface in the liquid immersion medium fluid.
It is important that the liquid immersion medium fluid has a high transparency in exposure wavelength. In the 193 nm immersion lithography, ultra pure water having a refractive index of about 1.44 is expected as the most promising solvent. In the 157 nm immersion lithography, a test example in which a fluorine solution is used as the solvent has been reported. With an high refractive index of the liquid immersion medium fluid, it becomes possible to form a fine pattern by using a properly designed optical system.
In liquid immersion lithography, there is a concern that inconvenience in a resist process occurs due to a contact between the liquid immersion medium fluid and the resist film. Specific problems are exemplified as follows.
(1) With respect to many resist materials, a T-top shape occurs with a formed resist pattern, or defects after development increases.
(2) Decrease of uniformity occurs in the dimension or shape of the resist pattern within a wafer and/or within a shot.
(3) In the case where a liquid immersion medium fluid remains on a substrate after exposure, it can cause contamination and/or a breakdown of the exposure tool and/or the coating and developing device.
(4) In the case where bubbles occur in the optical path, it causes degradation of the optical image as a flare or aberration. Even if the liquid immersion medium fluid is degassed, relative movement of the liquid immersion medium fluid occurs on a step of a substrate or at an edge of the substrate, and an atmospheric gas in the exposure tool may be captured in the liquid immersion medium fluid.
In order to avoid a contact between the liquid immersion medium fluid and the resist film, it is examined that a cover material film is provided on the resist film. Even in the case where the cover material film has been provided, however, there is a possibility that all of the above-described problems occur, although their degrees are different from each other, in the case where the shield of the cover material film against the liquid immersion medium fluid is not sufficient; in the case where there occurs capturing of the resist film component dissolved by the solvent of the cover material into the cover material film; or in the case where there occurs deposition of particles onto a surface of the cover material film during coating and/or baking process. At least, there is a possibility that decrease of uniformity occurs in the dimension of the resist pattern within a wafer and/or within a shot.